Q.What is meant by interference of light? Write down the conditions of sustained interference. OR What is an optical fibre? Mention its uses. What is absolute refractive index?

Interference is the modification of intensity when two or more light waves overlap; for the resulting bright/dark pattern to stay fixed in time (sustained), the sources must be coherent.

Interference of light: When two (or more) light waves of the same frequency, travelling in nearly the same direction, superpose, the resultant intensity at different points is not simply the sum of the individual intensities — it varies from a maximum (constructive interference, bright fringe) to a minimum (destructive interference, dark fringe), depending on the phase difference between the waves at that point.

Conditions for sustained (i.e. time-independent, observable) interference:

1. The two sources must be coherent — they must maintain a constant phase difference at all times (in practice, obtained from a single source, e.g. by division of wavefront or amplitude).
2. The two sources must emit light of the same frequency/wavelength (monochromatic). …